In general, another second metal layer is plating on a metal steel sheet by electrical-chemical treatment or using a hot dipping bath which has a second metal for coating molten therein and then a steel sheet submerged therein.
A representative molten metal coating adopts galvanized steel sheet. That is, the steel sheet, to be galvanized, passes around rolls of a zinc pot roll filled with molten zinc.
Especially, a continuous galvanizing line (CGL) allows the steel sheet to continually pass around the zinc pot roll to have its surface coated with zinc.
A galvanized steel sheet was utilized as a construction material initially. However, recently the steel sheet has been more extensively used, for example, in various assortments of electronic products or automobile materials due to endurance of harsh process conditions and mass producibility thereof.
Therefore, with its use broadened to electronic products or automobile materials where surface quality is crucial, the steel sheet faces an increasingly higher standard for better hot dip coating, i.e., to enhance surface glossing, corrosion resistance, weldability or coatability.
FIG. 1 illustrates a roll of a zinc pot in a continuous galvanizing line (CGL) and internal equipment thereof.
That is, as shown in FIG. 1, a steel sheet 200 is continuously supplied from a pay-off reel (not illustrated) to be connected to a heating chamber and induced to the zinc pot (bath) 220 filled with molten zinc 222 through a snout 210 immersed below a molten level of the zinc pot 220.
Also, the steel sheet 200 starts to pass perpendicularly toward an upper part of the zinc pot by virtue of the zinc pot roll immersed in the zinc pot 220, i.e., a sink roll 230 and stabilizing rolls 240 disposed just there over.
That is, the molten zinc 222 filled in the zinc pot 220, is attached and coated onto a surface of the steel sheet 200.
Here, the steel sheet 200 that passes around the sink roll 230 passes between the pair of stabilizing rolls 240 disposed just there over and these stabilizing rolls serve to prevent warping of the steel sheet.
Then, the steel sheet 200 passes between a pair of air knives 250 disposed just above the molten level of the zinc pot, thereby adjustably attached with molten zinc.
Then, the steel sheet is cold solidified and wound into a coil through a tension reel (not illustrated).
Therefore, the sink roll 230 and stabilizing rolls 240 of FIG. 1 are submerged in molten zinc (molten metal) of the zinc pot (hot dipping bath). The sink roll 230 and stabilizing rolls 240 guide a direction of the passing steel sheet 200 or corrects bending of the steel sheet which occurs when the steel sheet submerged in a high temperature zinc pot (zinc bath) emerges over the molten level.
For example, as shown in FIG. 1, the steel sheet 200 is connected to the heating chamber and passes through a sealed tube type of snout 210 which is submerged in the molten level of the zinc pot. Subsequently, the steel sheet 200 changes its motion toward a perpendicular upward direction due to the sink roll 230. Then the pair of stabilizing rolls 240 disposed just over the sink roll 230 imposed pressure on front and back surfaces of the steel sheet 200 transferred there between.
Accordingly, the sink roll 230 and the stabilizing rolls 240 suppress warping, distortion, inflection or vibration of the hot steel sheet 200.
Yet the zinc pot roll (sink roll 230 and stabilizing rolls 240) is different from a driving roll. That is, the zinc pot roll is immersed in the zinc pot 220 filled with hot molten zinc having a temperature of about 450° C. to 460° C. to operate while the driving roll runs at a room temperature.
Also, the zinc pot roll is not driven by a separate driving source, and rotated by a force of the transferred steel sheet, contacting the steel sheet without being powered. Therefore, tension load varies with thickness and width of the passing steel sheet 200.
Meanwhile, referring to FIG. 1, the sink roll 230 is spaced apart from a deviation roll (not illustrated) disposed over a cooling zone at a distance of 50 m to 60 m. This range of distance leads to loss of tension or inaccuracy of transfer of the steel sheet, thereby causing the steel sheet to vibrate severely.
Accordingly, the stabilizing rolls 240 installed just over the sink roll 230 critically serve to correct vibration or deformation such as inflection of the steel sheet when transferred.
However, the steel sheet produced by the steel manufacturer has various thickness and width ranging from 0.4 mm to 2.3 mm and 800 mm to 1860 mm, respectively. Thus one of the stabilizing rolls 240 (left side of FIG. 1) is fixedly disposed and the other one (right side of FIG. 1) is movably disposed.
For example, as shown in FIG. 2, one of the stabilizing rolls 240a is fixed to a rig 250, and the other one 240b is associated with a movable arm 260 which moves forward and backward by a driving source such as a hydraulic cylinder and a driving motor and installed on the rig so as to properly correct the steel sheet in accordance with thickness and width thereof.
Moreover, the sink roll 230 is engagingly disposed under the rig 250.
Thus, the rolls 230, 240a and 240b are introduced into or ejected from the zinc pot integrally with the rig 250.
But as shown in FIGS. 1 and 2, the stabilizing rolls having a smaller diameter and greater length than the sink roll are immersed in the molten metal 222 having a high temperature of at least 450° C. when rotating. Consequently, the stabilizing rolls are bent and deformed by tension of the steel sheet 200 and have a central line of the roll shaft inclined.
For example, FIG. 3 is a schematic diagram for analyzing actual load distribution of the stabilizing rolls 240 which are submerged in the zinc pot during operation.
As shown in FIG. 3, the stabilizing rolls with a shaft having a diameter of 50 mm to 70 mm keep a central shaft line of C1 before being affected by tension of the steel sheet and then have the central shaft line tilted toward C2 under influence of tension.
Then, FIG. 4 illustrates a conventional hot dipping bath roll in a continuous galvanizing line, especially stabilizing rolls 240 and a bearing part.
That is, as shown in FIG. 4, a bush 243 of the bearing part supporting a sleeve 242 fixed to a shaft 241 of the roll is fastened to a fixed bush housing 244. The fixed bush housing 244 is fixed onto a frame 245 associated with the rig 250 just described.
Accordingly, as shown in FIGS. 3 and 4, the stabilizing rolls in the conventional zinc pot tend to experience a tilt in their central shaft line owing to bending resulting from tension of the steel sheet. However, the bearing part cannot compensate for the inclination of the central shaft line.
For example, FIG. 4 illustrates a structure in which the bush 243 is formed integral with the fixed bush housing 244. Here, the sleeve 242 of the roll shaft 241 is in local contact with the bush 243 and thus easily inserted thereinto.
Therefore, the roll shaft rotated increases local friction between the sleeve and bush so that the roll itself fails to rotate smoothly.
Moreover, stiff rotation of the roll shaft 241 causes a skid in a contact area between the stabilizing rolls 240 and the steel sheet 200.
As a result, the slide between the steel sheet and the rolls leads to surface defects of the steel sheet.
The zinc pot rolls, i.e., zinc pot rolls such as the sink roll or stabilizing rolls rotate only by friction between the steel sheet and the roll surfaces in the zinc pot 220 filled with molten metal without a separate driving source. Thus, the stabilizing rolls rotate with reduced rotation force owing to viscous resistance or sliding friction of the roll bearing part.
Typically, viscous resistance and sliding friction are proportionally increased by relative velocity of an object. Therefore, the zinc pot roll is rotatable only when friction between the steel sheet and the roll surface is greater than the sum of viscous resistance and sliding friction of the bearing part.
In an actual assembly line of the steel manufacturing, the steel sheet 200 passes the zinc pot faster to increase production speed of the hot dip coated, i.e., galvanized steel sheet. But resistance friction as just described surpasses rotation force, thereby causing a skid between the roll and steel sheet. That is, the steel sheet to be galvanized in the zinc bath can hardly move faster in the actual assembly line.
Consequently in the zinc pot roll, as shown in FIG. 3, it is imperative to correct bending of the rolls and prevent local contact between the sleeve and bush.
Although not illustrated in a separate drawing, another conventional bearing part of a hot dipping bath roll is disclosed in U.S. Pat. No. 5,549,393 to overcome problems associated with such a conventional hot dipping bath roll.
For example, although not illustrated in a separate drawing, FIG. 6 of the U.S patent teaches a hot dipping bath roll which includes a retainer, a sleeve and a bearing ring. The retainer has a spherical annular convex surface. The sleeve has a frusto-conical concave annular surface which movably supports a left annular outer surface of the retainer and is supported by a housing. The bearing ring has an annular concave frusto-conical surface which movably supports a right annular outer surface of the retainer.
Therefore, in the roll according to the aforesaid document, the spherical annular convex surface of the retainer is in point contact with the frusto-conical concave annular surface of the sleeve (by the frusto-conical concave annular surface). Thus, practically, a contact area between the retainer and sleeve sustains load intensively, thereby resulting in local deformation.
In addition, sliding friction arising between the retainer and sleeve brings about local friction, thereby aggravating vibration of the roll rotated.